1. Technical Field
The present invention relates generally to a power control system for a centrifugal machine, and more particularly to a power control system for an electric motor driving a rotor of a centrifugal machine, which is designed to reduce harmonic components contained in the current supplied from or returned back to a power source during quick acceleration or deceleration of the rotor of the centrifugal machine for improving current waveform distortion and a power factor.
2. Background Art
FIG. 23 shows a conventional power control system for an electric motor of a centrifuge.
The power control system shown includes a power running thyristor bridge circuit 2, a regenerating thyristor bridge circuit 3, an inverter circuit 5 for driving an electric motor 4, a power factor-improving choke coil 6, and a smoothing capacitor 7. The power running thyristor bridge circuit 2 and the regenerating thyristor bridge circuit 3 are, as clearly shown, electrically oriented in directions reverse to each other in connection with an AC power supply 1 to have a bi-directional power-converting function.
A CPU 8 provides, as control signals, gating pulse signals to gates 2G and 3G of the power running thyristor bridge circuit 2 and the regenerating thyristor bridge circuit 3 through date-pulse delay timer LSIs 9 and 10, drivers 11 and 12, and pulse transformers 13 and 14. A timer LSI provides an oscillation output to an on-off pattern generating logic circuit (PLD) 16, which is, in turn, amplified by a base driver 17 to provide a base current to transistors of the inverter circuit 5.
A V-sensor 18 measures the voltage of the AC power supply 1 to provide a signal indicative thereof to a zero-cross circuit 19. The zero-cross circuit 19 then provides a reference phase signal to the CPU 8. When it is required to have a rotor 20 accelerate to a fixed speed through the motor 4, the CPU 8 phase-controls the power running thyristor bridge circuit 2 to perform PAM control for regulating a charged voltage in the smoothing capacitor 7 for V/F control of the motor 4. On the other hand, when it is required to have the rotor 20 decelerate through the motor 4, the CPU 8 phase-controls the regenerating thyristor bridge circuit 3 to transmit electrical energy produced by the motor 4 and stored in the smoothing capacitor 7 to the AC power supply 1.
Accordingly, in order to increase the speed of a rotor having a relatively large moment of inertia, the above type of centrifuge power control system needs to phase-control a power running thyristor bridge circuit and perform the PAM control for regulating a charged voltage In a smoothing capacitor for the V/F control of an electric motor for an extended period of time, thereby causing ac current containing a great harmonic component and having a decreased power factor to flow through the motor. This great harmonic component leads to distortion of a voltage waveform of a power supply, which may adversely affect a precision electric instrument such as an analyzer disposed around the centrifuge. Additionally, since a power supply capacity is usually restricted to a given value, the supply current having the decreased power factor disadvantageously requires temporarily stopping an operation of auxiliary equipment such as a temperature control unit controlling the temperature inside the centrifuge, a vacuum pump, and a diffusion pump during acceleration of a rotor, or dropping the acceleration of the rotor for ensuring a power supply for the auxiliary equipment. Similarly, decreasing the rotor in speed causes a harmonic current to be returned from a regenerating thyristor bridge circuit back to the power supply, thereby degrading the performance of the auxiliary equipment.